It is known to use carbon dioxide as representative parameter for measuring biological activities. Such measurements of biological activities are used for example for identifying the presence of micro-organisms in a sample, for example blood. Likewise the biological or the chemical oxygen requirement (BOD, COD) can be determined in this manner. A further application example of CO2 measurement is composting of plastic materials in which the plastic materials are treated with micro-organisms and nutrient solution. Monitoring of the progress of the decomposition during composting is implemented by the change in the measured carbon dioxide concentration over a lengthy period of time of up to approximately 150 days.
Various methods for measuring have been proposed. According to a first method, a gas sample is withdrawn from a sample bottle. Next the CO2 concentration is determined by means of gas chromatography. This method is however very labour intensive, errors being able to occur during transference of the gas sample into the gas chromatograph. In addition, the atmosphere above the sample is affected by the gas sample withdrawal. Furthermore, the withdrawal syringe must be decontaminated and disposed of after each sample withdrawal.
In a further proposed method, a gas sample is withdrawn from the sample bottle by means of a closed pump system which contains a gas analyser. The atmosphere of the sample bottle is hereby likewise changed. During testing of a plurality of samples, the closed pump system must be decontaminated in a technically complex manner after each measurement.
It is known furthermore to detect the produced CO2 through the wall of the sample bottle. For this purpose, the sample bottle is placed in the radiation path of an infrared absorption measuring unit. The CO2 concentration is determined by the weakening of the radiation in a characteristic wavelength, for example 4.24 μm. In this method, there are however high demands made on the bottle quality with respect to wall thickness and to the material, which results in high costs.
In addition, the measuring result can be falsified by means of moisture being condensed-out. By means of agitation of the bottles, the bottle inner walls can be soiled which in turn impairs the measurement. Thus, quantitative measurements are only possible with high technical complexity and high costs.
Finally, it is proposed in EP 0 425 587 B1 to use optical sensors, for example based on fluorophores in order to measure the CO2 concentration. The corresponding sensitive membrane is thereby introduced into the container to be tested, for example on the base, on the wall or integrated into a measuring chamber which is in contact with the sample bottle by means of diffusion with the help of a cannula. The optical properties of the membrane are monitored externally. It is however disadvantageous that the optical properties of the sensor can be impaired by other gases (NH3, alcohols, . . . ) and that the long term stability of the sensors is inadequate.
With the known methods, long-lasting and trouble-free quantitative measurements of gas concentrations in closed or open systems are therefore implementable either only in an unreliable manner or technically only in a very complex manner.
The object therefore underlying the invention is to produce a method of the generic type and a device for implementing the method so that gas analyses and in particular quantitative measurements of gas concentrations can be implemented over a long period of time, in a trouble-free manner and economically.